1. Field of the Invention
The present invention relates to a composition for reflective film, a reflective film, and an X-ray detector.
2. Related Art
There has been developed a planar detector using an active matrix, as a detector for X-ray analysis of a new generation. In such a planar detector, when the irradiated X-ray is detected, captured X-ray images or real-time X-ray images are output in the form of digital signals.
Specifically, an image is obtained by converting X-rays into fluorescence by a scintillator layer, and converting this fluorescence into signal charges with a photoelectric conversion element such as an amorphous silicon (a-Si) photodiode or a charge coupled device (CCD).
Here, in order to enhance the resolution characteristics, there are available a technology for forming grooves in the scintillator layer by dicing or the like, and a technology for forming the scintillator layer in the form of a pillar-shaped structure by deposition according to a vapor deposition method.
Furthermore, in order to enhance the sensitivity characteristics by increasing the use efficiency of the fluorescence from the scintillator layer, there is known a technology of forming a reflective film on the scintillator layer, reflecting the fluorescence radiated to the side opposite to the photoelectric conversion element side, and thereby increasing the light that arrives at the photoelectric conversion element side (see, for example, Patent Documents 1 and 2).
As the method for forming such a reflective film, a method of forming a film on the scintillator layer using a metal having a high fluorescence reflection ratio, such as a silver alloy or aluminum; a method of applying a diffusion reflective material composed of a light scattering substance such as titanium oxide, a binder resin and a solvent, on the scintillator layer; and the like are generally known.
However, in the method of applying the diffusion reflective material, the reflective film shrinks to generate stresses in the substrate as the solvent is removed after the application. As a result, warpage of the substrate or peeling of the scintillator layer and the reflective film from the substrate occurs, and there is a risk that the luminance and resolution may be lowered. This tendency tends to be noticeably exhibited as the size of the substrate increases, and therefore, it is difficult in the conventional methods to deal with large-sized substrates.
In the method of applying the diffusion reflective material, since the binder resin is filled in the gaps between the light scattering substances such as titanium oxide, the light scattering effect of the light scattering substance having a high refractive index may be drastically reduced. That is, for example, in regard to the magnitude of the angle of refraction at the interface between titanium oxide (refractive index 2.7) and the atmosphere (refractive index 1.0) caused by the difference between the refractive indices, since the difference between the refractive indices of titanium oxide (refractive index 2.7) and the binder resin (refractive index approximately 1.6) is small, the angle of refraction at the interface per round is decreased. The reflection by a light scatter is achieved as the direction of light travel randomly changes the direction as a result of repeated reflection at the interface between the light scatter and the neighboring substance, and thereby the light scatter successfully performs the role as a diffusion reflector. Thus, a decrease in the angle of refraction at the interface ends up with an increase in the number of refractions required by the direction of light travel to change the direction randomly. In other words, this means that it takes a circuitous route for the fluorescence radiated from the scintillator to change the direction in the same manner. This circuitous route makes the fluorescence radiated from the scintillator to affect both the extension in the thickness direction and the extension in the plane direction of the reflective film. As a result, the circuitous route leads to an intensification of a decrease in the resolution due to an increase in the crosstalk between the pixels, or to a reduction in the fluorescence radiated from the scintillator inside the reflective film, thus bringing about a decrease in the luminance.